1/2cupchocolate chips(I used 1/4 cup milk chocolate and 1/4 cup semi-sweet)
Preheat oven to 400 degrees. In a microwave safe bowl, combine bittersweet chocolate and butter and heat in microwave in 30 second intervals, stirring after each interval until melted and smooth, set aside.
In a mixing bowl, whisk together granulated sugar and cocoa powder then add in flour, baking powder, baking soda and salt and whisk until well blended.
In a separate bowl or in the liquid measuring cup used to measure buttermilk, blend together buttermilk, vegetable oil, egg, egg yolk and vanilla extract until well blended.
Pour buttermilk mixture and melted chocolate mixture into flour mixture, and using a rubber spatula, quickly stir and fold just until combined. Fold in chocolate chips.
Divide batter among 12 (standard size) paper lined muffin cups, filling each cup nearly full.
Bake in preheated oven until toothpick inserted into center of muffin comes out clean, about 15 - 18 minutes. Cool on a wire rack and store in an airtight container.